The present invention relates to a clamping device capable of applying a calibrated clamping force for locking two pieces together temporarily.
In the aircraft construction industry it is often necessary to couple together two large pieces, each having an extensive surface which is positioned facing towards, and is clamped to, an extensive surface of the other piece, or a plurality of separate support surfaces, which are spaced apart, of the other piece. This necessity arises, for example, when carbon-resin components such as stiffened panels, spars, ribs, or the like have to be coupled to each other and/or to light alloy or titanium parts, where the coupling is particularly critical owing to the different degrees of finish of the coupling surfaces. The two pieces are initially brought together and positioned in a predetermined mutual coupling position, with the two corresponding surfaces abutting each other. The two pieces are then held together temporarily, by temporary coupling means, in a predetermined position substantially corresponding to the flight configuration in which the two pieces will subsequently be finally joined together. Before the two pieces are finally joined together, for example by means of rivets, threaded fasteners, or other special fastening members (such as Hi-Lok or Lockbolt fasteners), it is necessary to check for the presence of any gap (or empty space or void) between the facing surfaces of the two pieces, and measure its size, if present.
If there is a gap whose dimensions exceed certain tabulated values, the clamping process by which the two pieces are finally coupled will inevitably entail an unacceptable degree of forcing to bring the two pieces into direct contact, leading to the appearance of stresses caused by deformations created locally for the approach of the pieces. These stresses constitute a pre-loading which is added to the stresses to which the pieces are subjected in use. In extreme cases, if the coupling forces applied in the presence of a gap are excessive, fractures may occur in the pieces when an acceptable stress level is exceeded.
In order to avoid this, a solid filling insert (or “shim”) has to be interposed between the facing surfaces of the two parts, to fill the gap in such a way that, when the two pieces are made to abut each other, they can be joined together without the generation of appreciable additional local deformation and stresses.
In order to manufacture an insert or shim having a variable thickness corresponding to the distances between the facing surfaces of the two pieces to be joined, the thickness of the void or gap between the pieces must be measured while the pieces are both in their natural, or non-deformed, position in which they are subsequently to be finally joined together.
In the aircraft industry, some manufacturers stipulate that the gap must be measured while the two pieces are temporarily held together, by the application of standard clamping forces at certain points; for example, it is specified that the forces must have an intensity of 5 lb (22.24 N) and must be applied at the positions of pairs of aligned holes formed in the two parts to be coupled, where each pair is separated from the other by an interval of one foot (0.305 m). For the application of these temporary retaining forces, use is made of through holes with diameters slightly smaller than those of the final holes into which the final fastening members (usually rivets or bolts) will be inserted.